1. Field of the Invention
This invention relates to computers employing power management systems and particularly to circuits and methods for allowing a computer to enter a power conserving mode while executing a host signal processing modem.
2. Description of Related Art
Host signal processing (HSP) modems reduce the cost of providing modem functions to a computer system by utilizing the processing power of the central processing unit (CPU) of a host computer rather than including a dedicated digital signal processor (DSP) in modem hardware. For example, typical HSP modem hardware includes: an interface for connection to telephone lines; a digital-to-analog converter (DAC); an analog-to-digital converter (ADC); a buffer for temporary storage of samples representing signals transmitted and received on the telephone lines; and an interface for connection to the host computer. During operation with an active connection over the telephone lines, the HSP modem hardware periodically interrupts the host CPU which in response executes HSP modem software. The HSP modem software retrieves received samples from the HSP modem hardware and converts the received samples to data for a communications application. The HSP modem software also retrieves data from the communications application, converts the data into samples representing the signal to be transmitted on the telephone lines, and writes those samples to the HSP modem hardware.
In an HSP modem, the host computer executes software which performs many of the conversions performed by hardware in a conventional modem. Hardware in the HSP modem performs simple analog-to-digital and digital-to-analog conversions such as converting a received analog communication signal to a series of digital samples which represent amplitudes of the received signal. The host computer executes software which interprets the samples according to a communications protocol and derives the desired data from the samples. The host computer also generates a series of output samples which represent amplitudes of a transmitted analog communication signal in compliance with the protocol, and hardware of the HSP modem converts the output samples into the transmitted signal.
One disadvantage of an HSP modem can arise from the interaction of the HSP modem with a power management system such as often employed to conserve battery power in a portable computer. One type of power management system tracks a computer's activity and when the computer is inactive for a predetermined period, places the computer in a power-saving (sleep) mode in which the system clock for the central processing unit (CPU) is slowed or stopped. The power management system turns on or increases the frequency of the system clock to place the computer in a normal operating mode when the computer resumes activity. An HSP modem can effectively disable a power management system by maintaining periodic interrupts to the main CPU. The power management system senses the periodic interrupts as system activity and keeps the computer in a normal operating mode. This is appropriate during an active telephone connection because the system is actively processing information and probably should not be in the power-saving mode. But, when there is no active modem connection, the HSP modem and the computer system may not be performing any useful activity, and the computer should be place in power-saving mode to conserve power. The HSP modem can be turned off to stop the periodic interrupts and allow the system to enter sleep mode, but this comes at the cost of losing modem functions. For example, while there is no active connection, the HSP modem may be waiting for a remote device to call in and establish a connection. Turning off the HSP modem disables the ability to handle an incoming call. Accordingly, an HSP modem system is sought that allows a power management system to place a computer in a power-saving mode when there is no active connection and is still able to respond to incoming calls received while in the power-saving mode.